Chemical-mechanical planarization ("CMP") processes remove material from the surface of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is pressed against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry solution generally contains small, abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. The polishing pad is generally a planar pad made from a relatively soft, porous material such as polyurethane. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals in the slurry (chemical removal).
FIG. 1 schematically illustrates a conventional CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a slurry 44 on the polishing pad. An under-pad 25 is typically attached to the upper surface 22 of the platen 20, and the polishing pad 40 is positioned on the under-pad 25. A drive assembly 26 rotates the platen 20 as indicated by arrow A, or in another existing CMP machine the drive assembly 26 reciprocates the platen 20 back and forth as indicated by arrow B. The motion of the platen 20 is imparted to the pad 40 through the under-pad 25 because the polishing pad 40 frictionally engages the under-pad 25. The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier 30 to impart axial and rotational motion, as indicated by arrows C and D, respectively.
In the operation of the conventional planarizer 10, the wafer 12 is positioned face-downward against the polishing pad 40, and then the platen 20 and the wafer carrier 30 move relative to one another. As the face of the wafer 12 moves across the planarizing surface 42 of the polishing pad 40, the polishing pad 40 and the slurry 44 remove material from the wafer 12.
CMP processes must also consistently and accurately produce a uniform, planar surface on the wafer because it is important to accurately focus optical or electromagnetic emissions in precise circuit patterns on the surface of the wafer. As the density of integrated circuits increases, it is often necessary to focus the optical or electromagnetic emissions to within a resolution of approximately 0.35-0.5 .mu.m. Focusing the circuit patterns to such small tolerances, however, is very difficult when the distance between the emission source and the surface of the wafer varies because the surface of the wafer is not uniformly planar. In fact, several devices may be defective on a wafer with a non-uniformly planar surface. Thus, CMP processes must create a highly uniform, planar surface.
The planarity of a polished semiconductor wafer is a function of several factors, one of which is the distribution of slurry between the polishing pad and the wafer. The polishing rate, which is the rate at which material is removed from the wafer, depends in part on the volume slurry between the wafer and the pad. To maintain a uniform polishing rate across the surface of the wafer and produce a uniformly planar surface, it is desirable to distribute the slurry evenly across the whole surface area of the wafer.
Another factor affecting the planarity of a polished wafer is the compressibility of the pad. Soft pads conform to the general topography of the wafer and result in a surface that retains some of the topographical features of the unpolished wafer. Relatively incompressible pads, on the other hand, do not readily conform to the topography of the wafer; as a result, hard pads planarize high points on the wafer before reaching low points to produce a more uniformly planar surface on the wafer. Therefore, it is generally desirable to provide a hard polishing pad that enhances the distribution of slurry between the wafer and the polishing pad.
One desirable technique to enhance the distribution of slurry under the wafer is to provide a porous structure in the polishing pad that holds additional slurry slightly below the polishing surface of the polishing pad. Conventional porous polishing pads have a body made from a continuous phase matrix material and a filler material made from hollow spheres or closed cell foam. The continuous phase matrix material is typically made from a compressible polymeric material, and the hollow spheres are typically made from polymers. When the pad is cut or conditioned, the center of the hollow spheres and porous structure of the closed cell foam form pores in the pad. The porosity of a pad is controlled by the density of the filler material in the continuous phase matrix material, with a higher density of filler material resulting in a higher porosity of the pad. Thus, it is generally desirable to use a higher density of filler material. One problem with conventional porous polishing pads is that the hollow spheres and closed cell foam are compressible and do not reinforce the continuous phase matrix material to provide a sufficiently hard polishing pad. The use of a higher density of filler material to provide high porosity accordingly results in a compressible pad that conforms to the topography of the wafer. Therefore, it is difficult to provide a polishing pad having both high porosity and substantial hardness.
Many techniques and structures have also been developed to increase the hardness of polishing pads. Typically, glass particles or fibers are added to the matrix material to reinforce the pad and increase its hardness. Glass reinforced polishing pads, however, do not have adequate porosity because glass is not readily soluble in solutions that may be used with polyurethane. Therefore, conventional polishing pads with glass particles and fibers do not distribute the slurry uniformly across the surface of the wafer.
In light of the problems associated with conventional porous and hard polishing pads, it would be desirable to develop a relatively hard polishing pad that has a sufficiently porous planarizing surface.